The invention relates to a screw anchor for fastening add-on parts in concrete or brickwork by means of a shaft, which encompasses a thread. Such a screw anchor is known from DE 10 2006 003 172 A1, for example.
As a general rule, a screw anchor is a one-piece component, in the case of which the thread is rolled onto the shaft of the screw anchor as external thread. The thread is to have a very high degree of hardness, so as to provide for a problem-free grooving of an internal thread into a pre-drilled hole in concrete or brickwork. Steel, which can be hardened and which has high carbon content, can thus be used as material. Such steels can only be recast with difficulty. The thread rolling thus proves to be very expensive.
In the assembled state, the shaft of the screw anchor supports high loads and is thereby permanently stressed so as to be subjected to tension and bending. Ductile, low carbon construction steel is considered here as material.
In the event that a corrosive attack is to be expected, as in the case of applications of the screw anchor in the outside caused by climatic influences, corrosion-resistant steel is to be used.
The mentioned demands are partly conflictive and can thus only be fulfilled by means of very few, expensive materials.
A screw anchor comprising a shaft, the external thread of which is formed by a first spiral, which is wound onto the smooth outer periphery of the shaft, is known from U.S. Pat. No. 1,181,972, wherein a constant distance between the coils of the first spiral from a second spiral, which is also wound unto the smooth outer periphery of the shaft and which has a lower degree of thickness than the first spiral, is ensured.
The invention is thus based on the object of embodying a screw anchor of the afore-described type in such a manner that it can optimally fulfill the demands during operation in response to reduced production costs of each of its parts.
Thanks to the division of the screw anchor into two parts according to the invention, namely into the load-accommodating shaft on the one hand and into the thread forming spiral on the other hand, materials can be chosen for the production of these two parts, which are in each case optimally adapted to the specific demands, so that these demands can be optimally fulfilled, without having to make the compromise, which is currently required and which was a sequel of selecting a single material for the screw anchor. Surprisingly, the production is also simplified, because a thread rolling of a material, which is difficult to shape, becomes unnecessary.
According to the invention, the spiral is connected to the outer periphery of the shaft in a positive manner, for example an embodiment, in which protrusions at one part, e.g. the spiral, interact in a positive manner with recesses at the other part, e.g. at the shaft.
In another type of positive connection, the spiral is inserted into a spiral groove, which is prefabricated to the periphery of the shaft.
The positive connection according to the invention does not require any additional parts or expensive operating processes for establishing the connection.
In the event that a corrosive attack is not to be expected, the shaft can consist of a material comprising high tensile strength, flexural strength and fatigue strength, such as common construction steel and the spiral can consist of a material comprising a high degree of hardness, such as hardenable steel comprising a high carbon content.
In the event that provision is made for use of the screw anchor in the outside, where corrosive attack is to be expected due to changing climatic influences, the shaft can preferably consist of a corrosion-resistant material, such as corrosion-resistant steel, and the spiral can be embodied in two parts, namely comprising a cutting part, which consists of two or three coils and of a material having a high degree of hardness, such as a steel comprising a high carbon content, and comprising a supporting part consisting of a corrosion-free spring steel.
The invention can also be applied to a screw comprising a shaft and internal thread, in the case of which the internal thread is formed by means of a separate spiral, which can be inserted into a blind hole or clearance hole in the shaft and which can be fastened in a similar manner as described above, e.g. in a spiral groove at the inner periphery of the blind hole.
The invention will be explained below more extensively at hand of schematic drawings of exemplary embodiments comprising further details.